Grasvally, Zoetveld - Bushveld Complex
Limpopo (Northern) Province, South Africa
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The historic Grasvally and Zoetveld chromite deposits are located towards the southern termination of the Northern Lobe of the 2050 Ma Bushveld layered mafic complex in South Africa, 16 km south of the Mogalakwena (previously Potgeitersrus) platinum mine at Mokopane (#Location: 24° 20'S, 28° 59'E).
For details of the Bushveld stratigraphy, mineralisation and chromite bearing reefs, see the Bushveld Complex record.
The Rustenburg Layered Suite of the Bushveld Complex is conventionally divided into five stratigraphic zones from the base, based on modal mineralogy, namely the: i). Marginal Zone norites; ii). Lower Zone pyroxenites and harzburgites; iii). Critical Zone chromitite-pyroxenite-norite cyclic units; iv). Main Zone gabbro-norites; and v). Upper Zone anorthosites, ferrogabbros and magnetitites.
The northern lobe is characterised by the local development of unusually thick (800 to 1600 m) sequences of Lower Zone lithologies, the apparent absence of the Critical Zone, that is so well developed in the Eastern and Western lobes, and the variation of the mafic succession along strike (McDonald et al., 2005).
To the north of the NE-SW Ysterberg-Planknek Fault (half way between Grasvally and Mokopane), the PGE- and base metal sulphide-bearing Platreef forms the base of the Rustenburg Layered Suite (see the Mogalakwena record) and is developed as a 10 to 400 m thick package comprising texturally heterogeneous and variably altered pyroxenitic lithologies (e.g. Hutchinson and Kinnaird, 2005; Kinnaird, 2005; Holwell and McDonald, 2006; Hutchinson and McDonald, 2008).
Lower Zone cumulates are known below the Platreef (Yudovskaya et al., 2012), although it remains unclear if these represent isolated accumulations or a continuous layer. To the south of the Ysterberg-Planknek Fault, the magmatic succession contains locally developed Lower Zone harzburgites on and west of the farm Grasvally, forming the floor to a unique layered package, the 400 to 800 m thick Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member, which is overlain by Main Zone gabbronorites and then by Upper Zone rocks. South of Mokopane, Lower Zone harzburgites are only developed to the west of the north-south Grasvally Fault, underlying the GNPA member. East of the Grasvally Fault the Magaliesberg Quartzite Formation of the Transvaal Supergroup underlies the GNPA member instead (Smith et al., 2012).
A characteristic feature of the northern limb/lobe is the pronounced downward transgression of the mafic succession northwards from the ENE-WSW trending Thabazimbi-Murchison Lineament (which marks the southern limit of the northern limb) through the Palaeoproterozoic Transvaal Supergroup (van der Merwe, 2008), to a footwall that progressively to the north consists of interbedded quartzites and shales of the Magaliesberg Quartzite Formation, quartzites and shales of the Timeball Hill Formation, shales of the Duitschland Formation, the Penge Banded Iron Formation, to the Malmani Subgroup dolomites and Archaean basement granites and gneisses in the far north (e.g. Sharman-Harris et al., 2005; Holwell and McDonald, 2006; van der Merwe, 2008).
The north-east striking GNPA member outcrops over a distance of 30 km (Verbeek and Lomberg, 2005), and is divided into three units the;
Lower Mafic Unit (LMF) - dominated by fine- to coarse-grained mafic lithologies including gabbro-norites, norites, pyroxenites and feldspathic pyroxenites,
Lower Gabbronorite Unit (LGN) - predominantly homogenous, fine- to medium-grained gabbronorites which contain variable proportions of cumulus plagioclase, which are petrographically comparable to those typical of the Main Zone. The LGN is thought to represent a sill of fine grained Main Zone rocks (de Klerk 2005). The LMF is distinguished from the un-mineralised, homogeneous gabbronorites of the LGN by an increase in melanocratic lithologies, the development of two chromitites and elevated chromium values (900 to
75000 ppm; Smith et al., 2012); and
Mottled Anorthosite Unit (MANO) - which is distinguished from the underlying LGN and LMF by the marked increase in the proportion of plagioclase cumulates and the dominance of mottled and spotted anorthosites. Clinopyroxene typically forms less than 10 modal% in comparison to up to 30 modal% in the LMF. Cyclic units with gradational boundaries, on a scale of tens of metres, of orthopyroxenite, norite, gabbronorite and anorthosite are common within the MANO (Smith et al., 2012).
Platinum-group element (PGE) mineralisation is closely associated with base metal sulphides and is confined to the LMF and MANO where PGE grades range from 1 to 4 ppm (3PGE+Au).
Towards the south of the North limb, a northward tapering wedge of the Lower Zones is thrust over the GNPA along the SW dipping Grasvally Fault. The lower plate of the GNPA directly overlies Transvaal Supergroup sediments to the east, while to the east of the fault, the Lower Zone wedge is overlain by a thinner development of the GNPA. The faulting and emplacement of the Lower Zone wedge 800 m higher in the sequence is the result of an early F1 north-south directed reverse faulting event within the Rustenburg Layered Suite (Hulbert and von Gruenewaldt, 1985).
In this wedge, the Lower Zone reaches a minimum thickness of 1600 m, dips at 27°W, and comprises 37 cyclic units (Hulbert, 1983), which range from 10 to 140 m in thickness. These cyclic units can be grouped into a number of collective lithologic units, one of which, the Drummondlea harzburgite-chromitite, contains the chromitites mined at Grasvally and Zoetveld. The Drummondlea harzburgite-chromitite is developed in the upper part of the Lower Zone and contains 7 cyclic units. From bottom to top, each cyclic unit comprises i). an olivine-chromite cumulate; ii). occasionally overlain by a chromite-olivine cumulate; iii). an olivine-orthopyroxene-chromite cumulate; iv). terminated by an orthopyroxene cumulate. Many cycles are incomplete or cut-off before all are formed (Hulbert and von Gruenewaldt, 1985).
The upper and lower chromitite are confined to harzburgites (olivine-chromite cumulates) in cyclic units 26 and 24 respectively, 620 and 690 m respectively below a platiniferous feldspathic pyroxenite that is situated 130 m above the base of the GNPA. These two layers have a separation in the western Grasvally and Zoetveld of 52 m, and 64 m in the northern part of the Grasvally mine. Other thin chromitites occur within the sequence and can be used a reliable markers. All of these layers have an irregular dip from 30 to 47°W, rotated from the normal dip of the Rustenburg Layered Suite of 27°W by faulting (Hulbert and von Gruenewaldt, 1985).
The chromitites that were mined at Grasvally and Zoetveld were restricted to this Lower Zone wedge, restricted to olivine rich rocks. The second generation faulting, F2, is a WNW-ESE directed reverse block faulting event, defining a number of separate blocks with progressively greater displacement to the west. As a result the chromitites to the west have been uplifted above the present surface and eroded. The third generation of faulting F3, is SW directed and resulted in blocks of Pretoria Group quartzite being upfaulted as wedges within the Rustenburg Layered Suite (Hulbert and von Gruenewaldt, 1985).
The two mined chromitites have a superior composition compared to those mined elsewhere in the Rustenburg Layered Suite. They were formed when ~66% of the Lower Zone had crystallised (Hulbert and von Gruenewaldt, 1985).
Thicknesses of the chromitites are very variable, In the southern and central portions of the Grasvally mine the upper an lower chromitites are vary from 114 to 124 and 63 to 91 cm in thickness respectively, while in the northern section they are 83 to 110 and 95 to 111 cm respectively. The chromitites comprise the following components, thicknesses and grades (Hulbert and von Gruenewaldt, 1985):
Upper chromitite package, Grasvally Central and Southern, and Zoetveld:
Disseminated hanging-wall - 43 cm @ 12 to 15% Cr2O3;
Upper Chromitite - 36 to 41 cm @ 43 to 46% Cr2O3;
Disseminated footwall - 25 to 30 cm @ 25 to 28% Cr2O3;
Upper chromitite package, Grasvally Northern:
Disseminated hanging-wall - 25 to 29 cm @ 12 to 14% Cr2O3;
Upper Chromitite - 25 to 27 cm @ 35 to 41% Cr2O3;
Disseminated footwall - 22 cm @ 9 to 25% Cr2O3;
Lower chromitite package, Grasvally Central and Southern, and Zoetveld:
Hanging-wall olivine pyroxenite - 150 cm @ 0.8 to 0.8% Cr2O3;
Lower Chromitite - 38 to 43 cm @ 42 to 45% Cr2O3;
Disseminated footwall - 25 to 28 cm @ 12 to 18% Cr2O3;
Lower chromitite package, Grasvally Northern:
Hanging-wall olivine pyroxenite - 24 cm @ 3 to 5% Cr2O3;
Lower Chromitite - 47 to 57 cm @ 37 to 44% Cr2O3;
Disseminated footwall - 24 to 30 cm @ 1.14% Cr2O3.
The main chromitites have Cr:Fe ratios of ~2.22 in the Lower and 2.46 in the Upper Chromitite, and contain 0.2 to 0.4 g/t PGE (Hulbert and von Gruenewaldt, 1985).
For other detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1986.
Record last updated: 20/11/2012
This description is a summary from published sources, the chief of which are listed below.
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References to this deposit in the PGC Literature Collection:
Hulber L J, Von Gruenewaldt G 1986 - The structure and petrology of the upper and lower chromitite layers on the farms Grasvally and Zoetveld, south of Potgietersrus: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 pp 1237-1247|
Smith J, Holwell D, McDonald I and Pearton T, 2011 - New observations on the nature and setting of PGE mineralization in the northern Bushveld Complex, south of Mokopane, South Africa: in Barra F. et al., (Eds) Lets Talk Ore Deposits, 11th Biennial Meeting, SGA 2011, September, 2011, Santiago, Chile Proceedings Volume, 2 pp. 654-656|
Smith, J. W., Holwell, D. A. and McDonald, I., 2016 - Precious and base metal geochemistry and mineralogy of the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex, South Africa: implications for a multistage emplacement: in Mineralium Deposita v.49, pp. 667-692.|
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